Programmable controllers are typically connected to industrial equipment such as assembly lines and machine tools to sequentially operate the equipment in accordance with a stored program. In programmable controllers such as those disclosed in the above cited patents, for example, the control program is stored in a memory and includes instructions which are read out in rapid sequence and executed to examine the condition of selected sensing devices on the controlled equipment, or to energize or de-energize selected operating devices on the controlled equipment contingent upon the status of one or more of the examined sensing devices.
The processor for these controllers is designed to rapidly execute programmable controller type instructions which in medium to large sized controllers includes not only instructions that manipulated single-bit input and output data, but also arithmetic instructions, file handling instructions, timers and counters, sequencers and other, more complex instructions. Such instructions have become quite standardized in the industry and they may be directly associated with elements of a ladder diagram which is easily understood by control engineers. Program panels such as those disclosed in U.S. Pat. Nos. 3,798,612 and 3,813,649 and in U.S. Pat. No. 4,070,702 have been developed to assist the user in developing and editing ladder diagram type control programs comprised of such programmable controller instructions.
To insure that the programmable controller can respond quickly to change in the status of sensing devices on the controlled system, it is imperative that the controller execute the control program repeatedly at a very high rate. The rate at which a programmable controller can execute the instructions in its instruction set, as well as the size of the control program, are the primary factors which determine the rate at which the programmable controller can repeatedly execute, or "scan", the control program.
While ladder diagram control programs ar particularly easy to create and edit for relatively small to medium scale control tasks, they become cumbersome and inefficient to use in large control tasks. Large ladder diagram control programs are difficult to understand, difficult to trouble shoot, and require a long time to execute.
As manufacturing control became more complex, greater demands were placed on programmable controllers. In addition to operating manufacturing equipment, programmable controllers have been asked to keep track of production data, such as production quantities, rates and defects, and report that data to a central host computer. Controllers have also been set up to perform other computational tasks which often tied up their processor for relatively long periods of time, delaying the execution of the manufacturing equipment control program. This delay could result in the controlled equipment not being supervised frequently enough for fail-safe operation. Furthermore, the reporting and computational tasks typically are not as time critical as the equipment control function.